Reedit: "A minimalist, elastic and read-only Reddit web-app client, that lets you create custom 'Channels' with up to 3 subreddits each." Probably the first web application I've used that doesn't make me long for something native. Fantastic work.

I don't think the main argument wasn't that it was impossible, it certainly is possible to engineer usable enough web apps. The problem people have boils down to a few things:

1) Given a set amount of resources, using HTML5 mean less productivity in certain scenarios.

HTML5 is a rather disjointed mash up of a bunch of technology, and the tooling around it is poor. The language (JS) isn't easily scalable to medium sized solutions and the performance takes a lot of extra work.

2) Integration with the host platform

It is harder to integrate with the underlying OS in a meaningful way. It's more difficult to appear native when you need to support a lowest common denominator of UI constructs. It leads to apps that look and feel foreign on every platform

3) The web is still in flux

There are various implementations of various states of emerging standards in every browser. Some browsers implement the Working Draft, some implement the Candidate Recommendation, etc. which lead to pretty subtle and some not so subtle differences in syntax and behavior.

This means there are complex feature matrixes to consider when deciding on which technologies to use. This makes it harder to utilize more modern layout models, for example.